A current area of development in wireless communications systems involves the use of transmitters and receivers having multiple antennas. Generally, these are known as multiple-input multiple-output (MIMO) systems and offer increased peak data rate, spectral efficiency, and quality of service though the use of a plurality of parallel data streams.
Relative to other wireless technologies, MIMO may enable substantial gains in both system capacity and transmission reliability without requiring an increase in frequency spectrum resources. By increasing the number of transmit and receive antennas, the capacity of a MIMO channel may be increased while also reducing the probability of all sub-channels between the transmitter and receiver fading simultaneously. However, recovery of transmitted information in a MIMO system may be relatively more complex and correspondingly benefit from accurate characterization of the channel. Accordingly, any of a variety of channel models may be used to tailor aspects of the operation of the transceiver to help optimize performance. Further, the characteristics of the channel may vary over time, which may warrant selecting different models to more accurately characterize the channel. In particular, operation in multi user (MU) MIMO mode may be relatively sensitive to the channel statistics. It would therefore be desirable to determine channel statistics in order to allow improved operation of the system.
Another aspect of the operation of MIMO systems is the capability to operate in a variety of modes. MIMO systems can be divided into operational classes including single-user MIMO (SU MIMO) and multi-user MIMO (MU MIMO). Further MU MIMO modes may be subdivided depending upon the number of users receiving the transmission. For example, MU2 may refer to a MU MIMO mode involving a simultaneous transmission to two users, and more generally, MUn may involve transmission to n users. An objective of SU MIMO operation may be to increase peak data rate per terminal, whereas an objective in MU MIMO may be to increase sector (or service cell) capacity. Operation in each of these classes has corresponding advantages. For example, SU MIMO exploits spatial multiplexing to provide increased throughput and reliability, while MU MIMO exploits multi-user multiplexing (or multi-user diversity) to further gains in capacity. Additionally, MU MIMO benefits from spatial multiplexing even when employing a receiver having a single antenna. As such, it would be desirable for the MIMO system to select from the available operating modes in order to improve overall performance of the system.
Due to the complexity associated with providing multiple transmit streams having adjusted phase and amplitude, MIMO systems rely on having accurate and current channel state information (CSI). In a closed-loop beamforming system, the channel may be estimated using a sounding protocol. By sending a known pattern of information, characteristics of the signal appearing at the receiver may be used to determine the CSI, which is then fed back to the transmitter. However, the transmission of the sounding signal represents overhead that may limit the overall throughput of the system. Since the benefit of providing current CSI by performing more frequent sounding protocols is offset by the increase in overhead, it would be desirable to select a sounding interval that optimizes performance.
This disclosure satisfies the goals identified above as well as other goals and needs.